Ultrastructural changes in the caudal neurosecretory cells of the trout Salvelinus fontinalis in relation to external salinity

Abstract

Ultrastructural changes in the caudal neurosecretory cells of the teleost Salvelinus fontinalis were studied when fishes were exposed to fresh water, deionized water, and seawater. In freshwater-adapted fishes, most cells presented a pattern of reduced synthetic activity of proteinaceous secretory material. The rough endoplasmic reticulum and the Golgi complex were moderately elaborated whereas secretory material was rare in the cell bodies. The nucleus appeared slightly irregular, with masses of heterochromatin. Exposure to deionized water for 3 days led to doubling of the mean size of the cell perikarya. The formation of neurosecretory material was strikingly stimulated. The rough endoplasmic reticulum was highly developed, and the Golgi complex showed intense vesiculation of elementary secretory granules which were abundant in the cells. Numerous and large globules of secretory material were observed in the cytoplasm. Also, the enlarged nucleus was lobulated and contained homogeneous euchromatin. After a 7- or 10-day exposure, no sign of activation of cell secretory activity was found. Transfer from fresh water to 25% seawater for 1 day enhanced synthetic activity in certain caudal neurosecretory cells but the majority remained inactive. Progressive exposure to 100% seawater was accompanied by a decreased level of activity in all cells. These findings appear to provide further evidence for the involvement of the caudal neurosecretory system in osmo- (iono)-regulation in the fish.